C - RLC FM Signal Phase Shifter

The circuit demonstrates how two quadrature signals can be derived from frequency-modulated (FM) wave. The comparators are zero crossing detectors which simply convert the smooth analog signals from their respective input terminals into digital pulses. The input to the first comparator is the original analog frequency-modulated (FM) wave. The output is also an FM signal but this is now in digital form. The input to the second comparator is taken from the RLC network which creates a phase shift that is a function of frequency. At FM carrier frequency the phase shift is 90°, as the frequency varies the phase shift varies from 0° to 180°. The output of the second comparator resembles that of the first but these two output signals are not coincident in time as a result of phase shifting. The variation in phase difference of the two output signals can be utilized to demodulate a frequency-modulated (FM) wave. Note: For convenience, the output of the comparators are labeled Iout (In-phase out) and Qout (Quadrature out) although these two signals have a phase difference of 90° only at the carrier frequency.